The present invention relates to an electronic fuel injection apparatus for supplying an internal combustion engine with fuel intermittently.
It is known well in the art related to an automotive vehicle that an internal combustion engine is supplied with fuel intermittently by an electronic fuel injection control apparatus which calculates the required amount of fuel electronically from the measured values of the operating parameters of the engine. The electronic fuel injection control apparatus typically calculates the required amount of fuel in proportion to the amount of air sucked into the engine and in inverse proportion to the rotation speed of the output shaft of the engine at every rotation thereof. It is therefore a great advantage of this type of electronic fuel injection control apparatus that the mixture ratio of air and fuel supplied to the engine can be kept substantially at a desired constant value and that the amount of fuel supplied to the engine can be modulated within a short period of time after a change in the operation of the engine. It is a further advantage that, since the electronic fuel injection control apparatus prevents fuel from being supplied to the engine during the deceleration of the engine, fuel economy is enhanced.
Contrary to these advantages, when the engine is decelerated with the throttle valve thereof being closed or accelerated from a low rotation speed, the automotive vehicle is apt to be subjected to a periodic surge, a vibratory back-and-forth motion, which deteriorates the vehicle driveability. While the automotive vehicle is subjected to the surge, it is observed that the rotation speed of the engine changes greatly at a frequency which is one-fourth of the rotation speed in timed relation with the surge with both the amount of sucked air and the intake pressure of the engine remaining unchanged.
The periodic surge is analysed to result from a resonance between the change in the output torque of the engine and the mechanical structure of the vehicle. When the throttle valve is closed to decelerate the engine, for example, fuel supply to the engine is prevented to decrease the output torque or the rotation speed of the engine and is resumed thereafter to increase the output torque or the rotation speed of the engine. At the transition from the prevention to the resumption of the fuel supply, an accelerating force exerts abruptly on the vehicle in response to the increase in the output torque of the engine. This abrupt acceleration causes a back-and-forth vibration in the mechanical structure of the vehicle. This mechanical vibration starts to damp in proportion to the lapse of time. However, since the amount of fuel is calculated in inverse proportion to the rotation speed of the engine, the amount of fuel supplied to the engine increases and decreases in response to the decrease and increase in the rotation speed of the engine, respectively. Further, since the engine performs intake, compression, power and exhaust strokes sequentially to rotate the output shaft twice in the case of a four-stroke cycle type, there is a time delay corresponding to one rotation of the engine from the supply of air-fuel mixture in the intake stroke to the generation of the output torque in the power stroke. Provided that the increase and decrease in the output torque of the engine resulting from the increase and decrease in the rotation speed happen to be in timed relation with respective forward and backward vibrating motions of the vehicle at the one-fourth frequency of the rotation speed, it is noted that the rich and lean air-fuel mixtures are supplied to said engine in the intake stroke in which the vehicle is subjected to the backward and forward motions respectively, and therefore the output torque of the engine is increased and decreased in the power stroke in which the vehicle is subjected to the forward and backward motions respectively. Therefore the back-and-forth motion of the vehicle is not damped any more but makes a resonance with the change in the output torque of the engine.
It is effective as a countermeasure to design the electronic fuel injection control apparatus to calculate the amount of fuel such that the basic mixture ratio of air and fuel supplied to the engine becomes richer than the stoichiometric ratio. This is advantageous to prevent the surge of the vehicle, since the change in the mixture ratio relative to the basic rich mixture ratio is small enough to decrease the change in the output torque or the rotation speed of the engine. However setting the rich mixture ratio is not advantageous from the standpoint of fuel economy.
It is more desirable as an alternative countermeasure to decrease the change in the mixture ratio relative to the change in the rotation speed of the engine with the basic mixture ratio being predetermined at or leaner than the stoichiometric ratio.